Aldi buy in a batch of well specified computers and sell them as one of their weekly special offers. That explains why Aldi cannot supply you as they have sold all that they ordered. Medion may not supply an identical specified computer but have something fairly similar or even better; but it may be at a premium, as you will only be buying one computer instead of the hundreds of thousands which Aldi order.
If you can wait a little longer then I'm fairly sure that Aldi will have another computer fairly soon as it generates a lot of interest as well as money. If you can't wait try PC World which sometimes features Medion computers, but usually at a slightly higher price or lower specification than Aldi.